I have 3.31s and I avg around 21-22 but I'm usually going 80mph and very rarely do I get passed. I drive 60miles a day on the highway. If you slow down to 70 the mileage comes up but I just can't drive that slow and watching cars pass me by. :)
I had an 03 Cobra as well and while I think it's cheaper to make it go fast the new car wins in every other category especially if you daily drive it like I do.